The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus of the type transferring a toner image from a photoconductive element to a recording medium.
With an image forming apparatus of the type described, there is an increasing demand for the high reproducibility of halftone and thin lines and for easy editing and processing of image data. To meet this demand, an image forming apparatus of the kind using, for example, a laser beam modulated by a digital signal as writing light has been proposed and put on the market in various forms. This kind of image forming apparatus can write an image over only an extremely narrow width on a photoconductive element, e.g., the width is as narrow as about 70 microns when the image density is 400 dots per inch. Hence, a change in the writing position on the photoconductive element directly translates into a change in the distance between nearby dots, resulting in noticeable irregularities in density on a reproduction. The irregular rotation of the photoconductive element and the vibration of the entire apparatus are the typical factors that cause the writing position to change, and various countermeasures have been proposed in the past. Taking the vibration of the photoconductive element as an example, a bearing which supports the shaft of the element is loose fit to facilitate the replacement or the like of the element and, therefore, a clearance exists between the shaft and the bearing. It follows that a plurality of photoconductive elements as often included in a particular type of image forming apparatus fail to be parallel to one another, i.e., their axes are inclined.
To eliminate the influence of the above-mentioned clearance, the shaft of the photoconductive element or the bearing may be urged in one direction to compensate for the clearance, as disclosed in Japanese Utility Model Publication No. 185151/1988 and Japanese Patent Laid-Open Publication No. 94356/1989. However, the problem with this scheme is that an adjusting screw for urging the bearing in one direction is accommodated in a narrow space defined at the side and, therefore, not easy to operate. Moreover, even if the machining accuracy of the shaft and bearing is high, a single bearing cannot eliminate a small clearance and, therefore, causes the shaft to shake and tilt.
The bearing may be implemented by a ball bearing having an inner race, balls, and an outer race. Such a bearing is not desirable since the outer race, balls and inner race have clearances therebetween. Further, the shaft of the photoconductive element would be loose fit in the inner race. For example, assume that the outside diameter of the shaft is 12 millimeters, and that the tolerance thereof is f7. Then, since the inside diameter of a bearing generally has a tolerance of 0 to 0.05 millimeters, the clearance between the shaft and the inner race of the ball bearing is as great as 11 to 34 microns.
Among the clearances stated above, the clearance within the bearing may be eliminated by prepressurization, as has been customary in the machine tool and precision machine industries. When use is made of a slide bearing, the clearance within the bearing will be eliminated. However, when it comes to the clearance between the bearing and the shaft, simply reducing the clearance would make the assembly extremely difficult and, moreover, make the accuracy requirement of the shaft diameter extremely severe to thereby increase the cost. Assume that the clearance between the shaft and the bearing is 11 to 34 microns, and that the shaft of the photoconductive element is adjusted in position to compensate for the clearance. Then, since the distance between dots is 62.5 microns in an image forming apparatus having an image density of 400 dots per inch, a displacement of the shaft of 11 to 34 microns corresponds to 17 to 54 percent of the dot distance and, therefore, unavoidably appears on a reproduction as irregular densities.